KUWAIT, Aug 20 (Reuters) - Kuwait detained a prominent Sunni Muslim cleric at the Gulf state's airport on Wednesday after the United States included him on a sanctions list on suspicion that he is funneling money to militants in Iraq and Syria, a security source told Reuters.

Hajjaj al-Ajmi was detained at the airport on his arrival from Qatar, whose leadership supports Islamist groups such as Egypt's Muslim Brotherhood and gives refuge to prominent figures from Hamas and the Taliban but does not back the militant Islamists operating in Iraq and Syria.

The security source gave no further details about Wednesday's detention. Nobody at Kuwait's Interior Ministry was immediately available for comment.

In their push to crack down on financing of militants in Syria and Iraq, Kuwaiti authorities also briefly detained another prominent Muslim cleric on Sunday, Shafi al-Ajmi, who belongs to the same extended tribe as Hajjaj al-Ajmi.

Shafi al-Ajmi was later released without charge.

Kuwait has been one of the biggest humanitarian donors to Syrian refugees through the United Nations, but it has also struggled to control unofficial fund-raising for opposition groups in Syria by private individuals.

The Kuwaiti government, an ally of the United States, has stepped up its monitoring of individuals and charities suspected of collecting donations for militants linked to al Qaeda in Syria and in Iraq.

Last Friday, the U.N. Security Council imposed sanctions on six people suspected of financing Islamist militants, including two Kuwaitis, in a move aimed at weakening Islamic State and al Qaeda's Syrian wing, Nusra Front. (Reporting by Mahmoud Harby; Writing by Amena Bakr; Editing by Gareth Jones)